The present invention relates to a self-compensating spiral for a mechanical spiral balance-wheel oscillator in watchwork or other precision instrument, made of a paramagnetic alloy containing at least one of the elements Nb, V, Ta, Ti, Zr, Hf, as well as to a process for treating this spiral.
The existence of drift phenomena in the frequency of a spiral balance-wheel oscillator as a function of time is well known to watchmakers. Thus such an oscillator fitted with an as-manufactured spiral made of a ferromagnetic alloy will gradually see its frequency increase, resulting after one year in a variation in rate of the order of 10 s/day.
This is essentially explained by the fact that the manufacturing operations create perturbations in the crystalline structure of the spiral. For example, the plastic deformations resulting from wire drawing, from rolling or from forming the Breguet curve create defects in the crystal, such as vacancies, interstitials or dislocations. Excessively rapid cooling steps after heat treatment trap vacancies. Internal stresses are also generated by the plastic deformations or the sudden variations in temperature. Oxygen can diffuse into the crystal during heat treatments.
All these perturbations of the crystalline structure result in an elastic phenomena which slightly modify the Young""s modulus of the spiral. Usually, the an elastic phenomena result in a modulus lower than the purely elastic modulus.
Over the course of time, the defects created in the crystalline structure will migrate at room temperature by slow diffusion toward equilibrium positions that are more stable and the internal stresses will relax. These structure reorganization mechanisms will thus cause the an elastic phenomena which were perturbing the Young""s modulus slowly to disappear. In general, an increase in the modulus, tending toward its purely elastic value, is observed.
The relative variations due to the an elastic phenomena and to their disappearance are extremely small, of the order of 10xe2x88x924, but nevertheless clearly measurable on the rate of a watch since a relative variation in the modulus of 2.3xc3x9710xe2x88x925 corresponds to a variation in rate of 1 s/day.
In order to reduce this drift, it is common practice to carry out heat treatments, called oven treatments, which consist in heating the completed spirals at a moderate temperature, of between 100 and 250xc2x0 C., for a period of 6 to 24 h. These treatments make it possible to reduce the rate drift over the first few years to below 1 s/day, this being acceptable given the other perturbations caused by wearing the watch, such as knocks. The oven treatment has the effect of accelerating, by thermal activation, the process of stress relaxation and of diffusion of the defects toward their equilibrium positions.
The same drift phenomenon is found in the case of spirals made of paramagnetic alloys, especially Nbxe2x80x94Zr, albeit even more accentuated, as illustrated by the diagram in FIG. 1. Unlike what happens in the case of the spirals made of ferromagnetic alloys, in the case of such a spiral made of a paramagnetic alloy the same type of oven treatment does not allow the rate drift to be reduced to below approximately 5 s/day after one year, as the diagram in FIG. 2 shows, in which curve a) corresponds to a spiral treated at 170xc2x0 C. and curve b) to a spiral treated at 270xc2x0 C.
The impossibility of reducing to within acceptable limits, or even eliminating, the residual rate drift of spiral balance-wheel oscillators fitted with spirals made of a paramagnetic alloy, especially Nbxe2x80x94Zr, may lead one to believe that other mechanisms are modifying the torque exerted by the spiral, in addition to those described above in the case of the spirals made of ferromagnetic alloys.
Paramagnetic alloys have a very great affinity for oxygen. In the ambient air, a surface oxide film is formed, which passivates the alloy. The presence of this film, despite its small thickness of a few nm, perturbs the torque exerted by the spiral. This is because the thickness of the spiral is approximately 30 to 50 xcexcm and the relative variations in torque which perturb the rate of the oscillator are of the order of 10xe2x88x924 i.e. the order of magnitude of the ratio of the thickness of the oxide film to the thickness of the spiral. These considerations suggest that, on the one hand, rate drifts observed with spirals made of Nbxe2x80x94Zr are due to the modification of the oxide film in the ambient air over time, thereby explaining the reason why the conventional oven treatment by itself is unable to solve this drift problem, as it can do in the case of spirals made of ferromagnetic alloys.
Thus, it is possible to deduce from these considerations that the rate drift of a spiral balance-wheel fitted with a spiral made of a paramagnetic alloy, especially an Nbxe2x80x94Zr alloy, is due to the sum of two effects:
a) a volume effect created by the slow reorganization of the microstructure at room temperature. This effect is similar to that observed in the case of ferromagnetic spirals and can be eliminated by oven treatment;
b) a surface effect created by the oxidation and passivation of the surface layer in contact with the air.
The object of the present invention is to reduce, or even eliminate, the rate drift due to the abovementioned effect b).
For this purpose, the subject of this invention is a self-compensating spiral for a mechanical spiral balance-wheel oscillator in watchwork or other precision instrument. The subject of this invention is also a process for treating this spiral.
The growth of an oxide layer on the surface of the spiral also serves to passivate it so that it will no longer experience a drift in its torque over time, due to the slow formation of the oxide layer in the open air, as occurs otherwise. The advantage of the anodizing process for forming this oxide layer is that it can be carried out at low temperature, without interfering with the crystalline structure of the spiral. In addition, this anodizing treatment allows the thickness of the oxide layer to be adjusted very simply, completely safely and perfectly reproducibly, to the desired value. Furthermore, the thickness of this layer, and therefore the color of the spiral thus obtained, is perfectly uniform.